The present invention relates to a pre-coated sheet steel capable of providing an excellent cosmetic corrosion resistance and crater-free paint appearance. More particularly, it relates to a duplex plated sheet steel having a lower layer of Zn base alloy and an upper layer of Zn plating or Zn base alloy or composite plating. The pre-coated sheet steel will be particularly suitable for automobile exterior panels.
As the amount of road deicing salt used in North America and Europe has been increasing, various zinc alloy plated sheet steels have been developed and some of them are practically employed in order to protect automobile bodies more effectively against inside-out corrosion.
Furthermore, the demand becomes greater for a good protection against cosmetic corrosion of automobile exterior panels in recent years. The cosmetic corrosion initiates at a paint damage (i.e., nick of the paint film applied on the substrate of sheet steel) mainly caused by the attack of stones or sands used together with deicing salt.
If the substrate is a bare cold rolled sheet steel, red rust bleed occurs to form scab corrosion. In order to improve cosmetic corrosion resistance on the exposed surface, therefore, it is expected that red rust bleed through a nick of the paint film can effectively be suppressed if a pre-coated sheet steel having a plating layer capable of sacrificial protection such as zinc plated sheet steel is employed as the substrate. However, such pre-coated sheet steels are disadvantageous in that the corrosion rate of zinc accompanying the sacrificial protection is so rapid that corrosion of the zinc layer itself tends to spread under the paint film to the surroundings of the nick, resulting in creepage of the paint film. Fe-Zn alloy plated sheet steels can reduce such corrosion under the paint film, but they tend to develop paint defects called "craters" in cathodic electrophoretic coating usually employed in painting of automobile exterior panels.
On the other hand, Zn-Ni alloy plated sheet steels do not show sacrificial protection sufficient to prevent red rust bleed through a nick of the paint film, although they provide good protection against the under-paint film corrosion.
If it is attempted that the red rust bleed through a nick of the paint film is prevented by increasing the coating weight of the plating layer, the superior formability and weldability of thin plated sheet steels will be lost.
In view of these problems of the prior art single layer plated sheet steels, duplex plated sheet steels have been developed which consists of two different Zn base plated layers on a base sheet steel.
Duplex plated sheet steels which have a lower Zn-Ni alloy layer and an upper Zn alloy or Zn composite layer are known in the art. For example, as indicated by the compositions of the lower layer/upper layer, Japanese Laid-Open Patent Application No. 207194/1982 discloses duplex plating of Zn-Ni/Zn-Fe (Fe=5-30%), Japanese Laid-Open Patent Application No. 145996/1982 discloses that of Zn-Ni/Zn-Fe-Ni or -Co (Fe.gtoreq.15%, Ni or Co=0.5-8.5%) and Japanese Laid-Open Patent Application No. 70291/1982 discloses that of Zn-Ni/Zn-Cr (Cr=0.005-0.5%).
However, these prior art duplex platings are still unsatisfactory. For example, in the case of duplex plated sheet steels having an upper layer of Zn-Fe alloy, if the upper layer is an alloy of low Fe content, the corrosion rate of the duplex layer will become higher as discussed in the aforementioned Japanese Laid-Open Patent Application Nos. 207914/1982 and 145996/1982. Therefore, these Japanese applications teach that the upper layer should consists of a Zn-Fe alloy of relatively high Fe content. However, as the Fe content increases, the resulting alloy layer contains more Zn-Fe alloy phases. When an electrophoretic coating is applied on such plating, paint flaws called "craters" are often observed on the paint film after the electrophoretic coating and even after finish coating, deteriorating the film appearance significantly.
When the upper layer is a Zn-Cr alloy plating, the corrosion resistance of the duplex plating is improved with an increase in the Cr content of the alloy as described in Japanese Laid-Open Patent Application No. 70291/1982. However, Zn-Cr alloys containing more than 0.5% Cr render the plating appearance inferior so that the Cr content of the alloy is limited to 0.5% or less in this Japanese application. As a result, the improvement in corrosion resistance obtained by the use of a Zn-Cr alloy plating is also limited.
Japanese Laid-Open Patent Application No. 38494/1981 discloses a duplex plated sheet steel having a lower layer of Zn-Ni alloy (2-20% Ni) and an upper layer of Zn wherein the ratio of the film thickness of the lower layer to the total film thickness of the lower and upper layers is not greater than 1:5 (i.e., the ratio of the film thickness of the lower Zn-Ni alloy layer to that of the upper Zn layer is not greater than 1:4). The duplex plated sheet steel disclosed in this Japanese application is described as having good corrosion resistance, but the application does not teach anything about cosmetic corrosion resistance thereof. However, the cosmetic corrosion resistance of such duplex plated sheet steel is expected to be rather poor because the duplex plating disclosed has a thick Zn upper layer on a thin lower Zn-Ni alloy layer. The thin plating of Zn-Ni alloy as disclosed in the above Japanese application is not sufficient to improve the corrosion resistance. On the other hand, the upper Zn plating layer is effective for sacrificial protection of the base steel surface, but a thick layer of Zn will result in the creepage of paint film due to excessive dissolution of Zn under the paint film if the film is damaged to form nicks. Thus, a combination of a thin Zn-Ni alloy plating and a thick Zn plating is disadvantageous with respect to resistance to cosmetic corrosion at nicks of paint film.